1. Field of Invention
The present invention relates to semiconductor integrated circuits (driver ICs) that drive display devices, such as LCDs (liquid crystal displays; liquid crystal panels), and more particularly to semiconductor integrated circuits that drive display devices using a PWM (pulse width modulation) method.
2. Description of Related Art
Liquid crystal panels are widely used in display sections of small related art devices, such as wrist watches, portable telephones and the like. Color tone (hue and lightness) of an image displayed on a liquid crystal panel is determined by lightness of each color displayed based on each of image data of R (red), G (green) and B (blue).
For example, when 16-bit image data is used, generally, 5 bits are allocated to R (red), 6 bits to G (green), and 5 bits to B (blue). Accordingly, for G (green), lightness in 26=64 gradations can be displayed, and color tone in 216=about 65 k colors in total can be expressed. It is noted that image data of R (red) and B (blue) are used through changing their unit bit length from 5 bits to 6 bits, as the same circuit is shared.
Also, when 12-bit image data is used, the unit bit length of image data in each color is 4 bits, such that, for each color, lightness in 24=16 gradations can be displayed, and color tone in 212=4096 colors in total can be expressed. When the unit bit length of image data of each color is either 4 bits or 6 bits, and both of the image data are to be accommodated, the unit bit length of 4 bits is converted to 6 bits.
Display signals that are to be supplied to a liquid crystal panel may be generated using a PWM method in order to determine lightness of each color. For example, display with lightness in 64 gradations is performed through comparing image data having a unit bit length of 6 bits and data each having 6 bits which are counted up and outputted at a predetermined timing, making pixels of the liquid crystal panel to emit light until the two coincide with each other, and prohibiting the pixels of the liquid crystal panel from emitting light after the two coincide with each other.
FIG. 9 shows a part of the structure of a related art driver IC. As indicated in FIG. 9, the driver IC includes an image data conversion table 100 that converts the unit bit length of image data of each color in 4 bits or 5 bits into 6 bits, a RAM (random access memory) 101 that stores image data, a counter 102 that counts a clock signal inputted and outputs count values, a comparison data generation circuit 103 that outputs data corresponding to predetermined count values, and a coincidence detection circuit 104 that compares the image data outputted from the RAM 101 and comparison data that are successively outputted from the comparison data generation circuit 103 to detect coincidence thereof.
FIG. 10 is a schematic indicating count values that the comparison data generation circuit shown in FIG. 9 stores. The comparison data generation circuit 103 stores 63 kinds of count values indicated in FIG. 10. It is noted here that the comparison data generation circuit 103 outputs comparison data ranging from “000000” to “111111” while successively incrementing its value by 1 when inputted count values coincide with the count values stored. For example, the comparison data generation circuit 103 changes comparison data sixty three times in total, to generate one display signal, starting from an output of comparison data “000001” at the time of an input of a count value “0000001010” until an output of comparison data “111111” at the time of an input of a count value “1101001000.”